Related Stories

Meteorologists may be overestimating rainfall, according to a new study that shows small raindrops can fall faster than previously thought.

The study, published in the current issue of the journal Geophysical Research Letters looked at the speed and size of 64,000 raindrops that fell over Mexico City.

Until now, conventional wisdom has been that big drops fell faster than little drops. And, as they fell, drops reach a maximum speed known as terminal velocity - the speed at which gravity is matched by atmospheric drag.

Meteorologists have used these assumptions to determine how much rain is falling by using radar to measure the speed of the drops.

But researchers, led by atmospheric scientist Guillermo Montero-Martinez of the National University of Mexico, found some small drops fall faster than big drops and faster than their terminal speed.

Montero-Martinez and colleagues measured the speed of raindrops using optical array spectrometer probes, which measure the scattering of light. They also analysed the shape and size of the drops using a particle analysis system and mathematical modelling.

'Speedy' raindrops

The researchers found that as big drops fell, they caught up and merged with smaller drops, sometimes breaking apart into intermediate-sized drops.

For about half a second, these intermediate drops retained the speed of their bigger parent drops, falling faster than their terminal velocity.

While these 'speedy' raindrops have been detected before, they had been attributed to raindrops splashing on the instruments, an error that the researchers say they avoided by only measuring during relatively calm conditions.

According to Montero-Martinez, meteorologists use instruments such as disdrometers and Doppler radar to measure the velocity of raindrops and estimate rainfall intensity.

"Because drop velocity is related to [the drop's] size, if the instrument detects a drop falling at large velocity it can consider its size erroneously," he says. "It may detect more large drops than are real."

The researchers believe their finding could improve the accuracy of weather prediction.

"If we want to forecast weather or rain, we need to understand the rain formation processes and be able to accurately measure the amount of rain," says Montero-Martinez.

Raindrop shape

Dr Alan Seed of the Bureau of Meteorology in Melbourne says the research highlights the fact that rainfall is "very complicated".

"There is not necessarily a one-to-one relationship between fall speed and size, which is what we have been assuming up to now."

Seed says the Bureau is currently trialling two experimental instruments that measure the shape of raindrops, which will "help make more accurate radar estimations".

"We have two experimental dual polarametric radars in Darwin and Brisbane that can measure the shape of the rain drop and we use this to tell us if the radar is seeing snow, hail or rain," he says.

"Rain drops become flat like M&Ms because of the drag on them as they fall, hail stones tumble … and the radar thinks that they are spheres, while snowflakes [appear] as flat ice crystals.

Seed says this information will help meteorologists study thunderstorms and make more accurate radar rainfall estimates.